Underwater pipeline installations are known to be performed using a variety of equipment in prior art; all the known arrangements and equipment, however, have left much to be desired regarding the speed of pipe laying as well as the reliability and efficiency of the operation.
At least two of the most desirable requirements for accurate, speedy and economical underwater pipe laying are to maintain stability of the ship or vessel during pipe laying operations in spite of adverse conditions of weather, high seas and winds, as well as achieving a fair amount of accuracy as to the course of the laid pipeline as compared to a planned pipeline route. Arrangements using two vessels or ships in a parallel position located side by side and supporting certain pipe laying equipment in a region between the ships are known in the prior art; however, the handling of pipes and the welding of pipes to an existing pipeline, the treatment of the welded joint and other associated operations are at best inefficient and sluggish; the structural arrangements required to support and control a pipe laying rig in between the two ships are not only elaborate and unwieldy, but also very expensive. It is also impractical to control the course of the pipeline with any reasonable degree of accuracy using an arrangement of two ships side by side since it would involve coordination of the movement of both the ships as well as coordination of the movements of the hoisting and control booms and the associated paraphernalia. Other situations in prior art where only one vessel or ship is used are known, but again, the speed of operation, and the efficiency as well as reliability of the welding and guiding operations are at best only poor. Furthermore, in most of the prior art operations, a major factor contributing to lack of efficiency and lack of speed has been the manner of handling the individual pipe lengths before they were prepared for welding, and the handling during thc time they were actually welded to a free end of the pipeline. It has been found out by experimentation and experience that the manner of receiving the pipe lengths from an external supply source into a pipe laying vessel, the manner of storing the pipe lengths and the precise manner of subsequently handling the pipe lengths including the orientation of the pipe lengths throughout the time they are present on the pipe laying vessel prior to consumption play a vital role in influencing the efficiency and speed of the entire operation.
In particular, it has been found that if the individual pipe lengths are all received, stacked, stored and transferred during installation always with the longitudinal axes of all the pipe lengths in the same direction, i.e., with the axes of the pipe lengths never changing direction once the pipe lengths are loaded on the ship, the handling time for the pipe lengths and welded pipe-sections is considerably reduced as compared to a conventional arrangement where the pipe lengths are not always aligned in the same direction. As to the question of maintaining stability of the vessel or the ship during pipe laying, it has been found by the inventors that by far the most optimal position for the pipe laying boom is the center of the vessel which also happens to be least susceptible to swaying and all undesirable movements including a horizontal oscillatory movement of the vessel technically known by the term "yawing". With the pipe laying boom arranged in the center of the vessel, an accurate course for the pipe laying under the water is assured within normal tolerances allowable for the course of the vessel per se.
It is also advantageous to have a butt welder apparatus built into the pipe laying boom so that when pipe assemblies (comprising two or more shorter lengths of pipes welded on board end to end) are hoisted and positioned for pipe laying, such pipe assemblies can be welded in the hoisted position end to end with a free end of a progressively laid pipeline. It is also expedient to have some suitable built-in means for treating a welded surface of a pipe before being lowered into water, for minimizing corrosion of the pipe welded surface in service.